Instances and Schemas in DBMS

In this tutorial, we are going to learn about the instances and schemas of the database management systems (DBMS).
Submitted by Anushree Goswami, on June 10, 2019

1) Instances

What is the Instance? If we look towards it in real life, we refer instance as an occurrence of something at a particular moment of time. In Database Management system, there are a lot of changes occurring over time to the information rapidly within the moments as the data get inserted, manipulated and deleted in parallel. The collection of information stored in the database at a particular moment is called an instance of the database.

2) Schemas

What is Schema? In the database management system, the overall design of the database is called the database schema. Schema gets changed rarely, and if so, changes altogether. A database schema corresponds to the variable declarations together with associated type definitions in a program.

At any given instant, each variable has a particular value and the values of the variables in a program at a point in time correspond to an instance of a database schema.

To understand the concept of database schemas and instances, we can do it through the analogy to a program written in a programming language.

Types of databse schemas

There are several schemas in a Database system that is partitioned depending on the levels of abstraction:

  1. Physical Schema
    The physical schema describes the database design at the physical level.
  2. Logical Schema
    The logical schema describes the database design at the logical level.
database schemas

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The physical schema is hidden beneath the logical schema, and it can be changed easily without affecting the application programs.

If the application programs are not dependent on the physical schema then they are said to display physical data independence, and so if the physical schema got to change in any case then, the application programs are needed to be rewritten.

Including these, in a database, there are several schemas present at the view level and called subschemas that describe different views of the database.

By far, the logical schema is the most important among all schemas due to its tremendous effect on application programs, as the programmers construct applications by using the logical schema.

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